The discovery that carbon nanotubes can be grown in dense, aligned arrays has inspired the conceptualization of their use in a number of unique applications. For example, aligned carbon nanotube arrays have been proposed for use in supercapacitors, field emitters, transparent and aligned conductive films, adhesive tapes, membrane filters, and speaker devices.
Although aligned carbon nanotube arrays have drawn significant research interest, there exists a potential drawback for their implementation in the above applications and others. Conventionally-prepared carbon nanotube arrays are prepared by depositing a thin, insulating oxide layer upon a substrate, followed by deposition of a catalyst layer upon the oxide layer. The oxide layer supports the catalyst, maintains its activity and promotes the growth of carbon nanotubes. The general requirement of an intervening oxide layer between the substrate and the catalyst prevents the carbon nanotubes from becoming bonded directly to the substrate. Although simple techniques have been developed to transfer aligned carbon nanotubes from a growth substrate to a desired substrate, direct growth on a desired substrate would be a far more efficient process.
Furthermore, the oxide layer is not compatible with a number of substrates, so there is a process limitation on the types of substrates upon which carbon nanotubes can be grown. As a result, for a number of interesting substrates, direct carbon nanotube growth is not possible by conventional growth processes. For example, direct growth of dense arrays of carbon nanotubes on a carbon surface (e.g., graphite, carbon fibers or foil, or diamond) or a conducting surface such as, for example, a metal is not possible by conventional growth methods. Carbon fibers are a substrate of particular interest due to their well-established use in the aerospace and polymer composite industries. Direct growth of carbon nanotubes on carbon surfaces according to conventional growth methods typically results in low carbon nanotube yields, sparse growth and potential damage to the carbon surface by the catalyst.
In view of the foregoing, methods for direct growth of carbon nanotubes on a substrate in the absence of an intervening oxide layer would be desirable in the art. Such direct growth methods would facilitate production of carbon nanotube arrays having the carbon nanotubes strongly bound to the substrate. Furthermore, such direct growth methods would also advantageously facilitate growth on non-conventional substrates such as, for example, carbon substrates and metal substrates, which are of interest in a number of potential carbon nanotube applications.